The invention relates generally to a method of applying a treating liquid to a porous body, in particular a prebaked carbon component of an aluminium production cell, such as an anode block or a cathode block, by placing the body in a treating chamber and impregnating at least a part thereof with the treating liquid, this impregnation being assisted by the application of a pressure differential.
The invention also relates to an apparatus for carrying out this method and use of the apparatus for applying a treating liquid to a prebaked carbon component of an aluminium production cell.
The treatment of prebaked carbon components of aluminium production cells, such as anode blocks, cathode blocks or cell sidewalls to improve their resistance to the conditions prevailing in the cell has already been proposed.
WO 94/29200 (Manga niello et al.) discloses treating a prebaked carbon-based anode of an electrolytic cell for the production of aluminium, in particular by the electrolysis of alumina in a molten fluoride electrolyte, over its sides and top to improve the resistance thereof to erosion during operation of the cell by air and oxidising gases released at the anode, by immersing the anode in a boron-containing solution containing 5-60 weight % of H3BO3 or B2O3 in methanol, ethylene glycol, glycerin or water.
It was found advantageous to carry out this treatment with a heated solution, but this involved heating of the anode, which consumed large quantities of energy. Attempts were therefore made to carry out the process at ambient temperature because no special heating equipment would be required. Low temperature application however required the careful choice of solvents and surfactant agents in order to reduce the treatment time as far as possible.
For prebaked anodes, only the top and top side surfaces need to be protected, so it was suggested to dip the anode upside down into the solution. But this is impractical when the anodes are fitted with rods for connection to a suspension device which also serves as a current lead-in. Furthermore, it is inconvenient to treat the anodes first and then fix the suspension rods.
To overcome this difficulty it would be possible to dip the anode in the treating solution with the rodded top side up, and protect the bottom part of the anode by blocking its pores with a fugitive agent that prevents impregnation with the boron-containing compound, and can be removed afterwards. This however entails additional operations and careful selection of the fugitive agent.
To speed up the process, it was suggested to assist the impregnation by the application of a pressure differential, by pressure or vacuum. However, no practical way of doing this was disclosed.
It is known from WO 93/25731 (Sekhar/de Nora) to treat carbon-containing components of an aluminium production cell to protect them from attack by liquid elements, ions or compounds by applying a coating of a refractory boride from a slurry composed of particulate refractory boride in a colloidal carrier. Sometimes it is desirable to apply such coatings selectively to the parts of the components which will be exposed, but methods and apparatus for doing this remain to be developed.
WO 94/24069 (Sekhar) describes treating components of aluminium production cells by impregnating them with various colloidal agents. U.S. Pat. No. 5,534,130 (Sekhar) describes the protection of the cell sidewalls of aluminium production cells by impregnating them with agents based on aluminium phosphate. Again, it would be desirable to perfect ways of applying these methods to selected parts of the components in an efficient manner.
It is an object of the present invention to obviate the above-described problems and shortcomings of the available methods and apparatus.
It is another object of the invention to provide a method and apparatus of the above type in which anode blocks or other bodies can be treated without necessarily pre-heating the bodies, while assuring an effective impregnation of the treating liquid into the pores of the treated part, and enabling the treatment of large numbers of the bodies in an efficient manner.
A particular object of the invention is to provide a method and apparatus which can use a hot treating solution in a very efficient manner to treat selected parts of the body, without necessarily heating the treated body substantially above ambient temperatures (although it is contemplated that treatments with cold solutions and hot anodes are also possible).
The method and apparatus of the invention were developed specifically with a view to overcoming the above-mentioned difficulties encountered when treating rodded prebaked anodes, but the method and apparatus can be used to treat other components of aluminium production cells and generally any porous bodies that need to be treated over part of their surface.
A method according to the invention comprises the following steps.
First, the body to be treated, in particular an anode block, a cathode block or a sidewall for an aluminium electrowinning cell is inserted, with its part to be treated facing up, in the treating chamber.
At least one sealing member is then applied to the inserted body in such a manner as to isolate the chamber into two parts: an upper part of the treating chamber around the part of the body to be treated, isolated from a lower part of the treating chamber around a bottom part of the body which is not to be treated.
Next, the upper part of the treating chamber is filled with a treating liquid to cover the part of the body to be treated, and a pressure differential is applied to intake an amount of the treating liquid into pores in the part of the body to be treated so that the pores of the body become impregnated with the treating liquid. The pressure differential may be applied simply by evacuating the lower part of the treating chamber. Application of the pressure differential is usually continued until all of the pores of the part of the body to be treated are filled. The exact degree of penetration can however be chosen for any particular application, e.g,. stopping the application of the pressure differential before all the pores are filled, or continuing the pressure differential for some time after the pores have been filled.
After completion of the impregnation, treating liquid which has not been absorbed by the body but remains in the upper part of the treating chamber is removed from the treating chamber, before or after the sealing member or members is/are released to free the body, and the treated body is removed from the treating chamber.
The sealing member may be arranged to surround the body and fit around its sides. In this case, the sealing member(s) is/are preferably arranged to allow a loose fit around a body, permitting insertion and removal thereof into or from the treating chamber, or to provide a sealing fit around a body in the treating chamber. Adjustment of the sealing member(s) can for example be controlled hydraulically, pneumatically, mechanically or electro-mechanically.
Bringing the sealing member to the sealing position can for example be controlled by detecting when the body reaches a given position, and actuating the sealing member(s) to sealably engage with the body when the body has reached said given position.
The or each sealing member preferably comprises an elastomeric body which is elastically deformed to provide sealing contact when applied against a body to be treated.
Alternatively, the sealing member can be arranged to apply or be applied against the bottom face of the body to be treated, thus allowing treatment of the entire sides and top of the body, or at least the lower parts of the sides.
The treating liquid in the upper part of the treating chamber is usually maintained at a temperature well above that of the body to be treated, for example a temperature in the range 60xc2x0 to 120xc2x0 C. To maintain the treating liquid at a more-or-less constant temperature, it is necessary to heat it to compensate for heat loss due to contact of the treating liquid with the body.
Alternatively, different temperature differentials can be used, for instance a hot body can be dipped in a cool treatment liquid, or the body and the treatment liquid can be at the same temperature.
In one embodiment of the method, treating liquid is circulated by the following arrangement. Hot treating liquid is supplied from a reservoir to the upper part of the treating chamber and, after treatment of the body, treating liquid which remains in the upper part of the treating chamber is returned back to the reservoir. The treating liquid in the reservoir is stirred and heated to maintain a desired temperature. Components of the treating liquid can be added to the reservoir at a rate to compensate for consumption of the treating liquid in the treating process.
Another inventive aspect is a method of supplying the treating liquid which comprises providing a saturated solution of the treating agent at a given temperature, and deriving, from said saturated solution, a non-saturated solution of the treating agent. The non-saturated treating solution is then supplied to said upper part of the treating chamber.
For example, the non-saturated solution is obtained by increasing the temperature of the solution so that the non-saturated solution contains the treating agent at the same concentration, which corresponds to the saturation concentration at said given temperature, but is simply at a higher temperature.
The method advantageously comprises the following sequential steps:
1) Actuating the sealing member(s) to sealably engage with a body inserted in the treating chamber when the body has reached a given position.
2) Filling the upper part of the treating chamber with hot treating liquid.
3) Evacuating the lower part of the treating chamber.
4) If necessary, applying heat to treating liquid in the upper part of the treating chamber to compensate for heat loss due to contact of the liquid with the body.
5) Ceasing to evacuate the lower part of the treating chamber; and
6) Removing remaining treating liquid from the treating chamber, before, during or after releasing the sealing member(s) to allow removal of the treated body from the treating chamber.
All of these steps can be automated, allowing a very efficient treatment of the bodies with minimum heat losses.
The treating liquid is preferably a solution which impregnates the part of the body to be treated, in particular a liquid containing an oxidation retardant agent such as one containing at least one soluble compound of boron, phosphorous or silicon for improving the resistance to oxidation of the carbon.
Alternatively, the treating liquid may be a suspension containing particles which block the surface pores of the body to be coated. Such a suspension may contain a colloid selected from colloidal alumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia, monoaluminium phosphate or cerium acetate.
For certain applications, for example for treating the surface of cathode blocks, the treating solution may contain particulate refractory boride, such as TiB2, and/or aluminium powder, chips or cuttings.
The treated body may be a pre-baked anode of an aluminium production cell, which anode is rodded, i.e. has a rod attached to its top face for connection of the anode to a suspension device which also serves as a current lead-in. Using the method and apparatus of the invention, rodded anodes can easily be treated with their rodded side up, which greatly facilitates handling.
The treated body may alternatively be part of a sidewall of an aluminium production cell, or a cathode block of an aluminium production cell.
The invention also relates to an apparatus for applying a treating liquid to a porous body by the method set out above.
The apparatus according to the invention for carrying out this method comprises a treating chamber having at least one sealing member which is arranged to be applied to a body to be treated which is placed in the treating chamber. The sealing member or members is/are arranged so as to isolate an upper part of the treating chamber around the part of the body to be treated from a lower part of the treating chamber around a bottom part of the body which is not to be treated.
Means are provided for filling the upper part of the treating chamber with a treating liquid to cover the part of the body to be treated. These means can include a pump for pumping treating liquid from a supply reservoir.
Means are also provided for applying a pressure differential to intake an amount of the treating liquid into pores in the part of the body to be treated, in particular by applying a vacuum to the lower part of the treating chamber.
Lastly, means are provided for removing remaining treating liquid, which has not been absorbed by the body during the treatment, from the treating chamber. These means can include another pump for pumping residual treating liquid back to the supply reservoir.
The sealing member(s) may surround the body and fit around its sides, and is/are conveniently arranged to be adjustablexe2x80x94for example by hydraulic, pneumatic, mechanical or electro-mechanical controlxe2x80x94to allow either a loose fit around a body permitting insertion and removal thereof into or from the treating chamber, or a sealing fit around a body inserted in the treating chamber.
The or each sealing member may comprise an elastomeric body which is elastically deformable when applied against a body to be treated. Such bodies can apply against the bottom of the body to be treated when it is decided to treat the complete sides and top of the body or at least the lower part of the sides.
A detector can be provided which detects the position of a body introduced into the treating chamber and actuates the sealing member(s) to cause them to sealably engage with the body when the body has reached a given position.
When a hot treatment liquid is to be used to treat bodies at ambient temperature, the treating chamber can be provided with means for heating the treating liquid in the upper part of the treating chamber to compensate for heat loss due to contact of the liquid with the body. Other arrangements to achieve a thermal balance can be used, as appropriate.
An embodiment of the apparatus comprises a reservoir for treating liquid, from which hot treating liquid is supplied to the top part of the treating chamber and to which remaining treating liquid, which has not been absorbed by the body during the treatment, is returned from the upper part of the treating chamber. This reservoir comprises means for heating and means for stirring the treating liquid therein. A metering device can be provided for adding components of the treating liquid to the reservoir to compensate for consumption of the treating liquid in the treating process.
A preferred storage vessel or reservoir xe2x80x94which can be used for various treatmentsxe2x80x94comprises first and second compartments, the first compartment containing a reserve supply of the hot treating solution in contact with a mass of the treating agent at a temperature T1. In the first compartment, the treating agent is dissolved at a concentration which corresponds to the saturation concentration, at temperature T1. The second compartment contains a supply of non-saturated treating solution in which the treating agent is dissolved at the same concentration but at a temperature T2 above said temperature T1, or at the same temperature but a lower concentration. The vessel further comprises: means for maintaining the hot treating solution in each of the first and second compartments at the respective temperature; an outlet conduit for supplying the non-saturated hot treating solution from the second compartment of the vessel to treat a body or material; and a conduit for supplying hot treating solution from the first compartment to the second compartment to compensate for consumption/loss of the treating liquid by treatment of the body or material.
The apparatus preferably comprises a control device arranged to sequentially actuate:
1) Means for bringing the sealing member(s) to sealably engage with the body when the body has reached a given position;
2) Means for filling the upper part of the treating chamber with hot treating liquid;
3) Means for applying a vacuum to the lower part of the treating chamber;
4) If needed, means for applying heat to treating liquid in the upper part of the treating chamber to compensate for cooling of the liquid by contact with the body;
5) Stopping the application of the vacuum to the lower part of the treating chamber; and
6) Means for removing remaining treating liquid from the treating chamber before, during or after relaxing the means for engaging the sealing member(s) to allow removal of the treated body from the treating chamber.
An advantage of the above-outlined method and apparatus is that it is possible to treat large bodies such as prebaked anodes without a need to pre-heat them. However, the bodies can be pre-heated if required. In particular, it can be useful to locally pre-heat the top part of the bodies to be treated.
Another advantage is that the method and apparatus allow sequential treatments to be carried out on the top part of the same body, over the same area or over a different area, using the same treating liquid or different treating liquids/slurries.